Fuel bundles utilized in modern boiling water nuclear reactor cores have had their basic structural components changed in both dimension and design. Before beginning a discussion of these structural components, a brief review of their conventional orientation is useful.
Fuel bundles in boiling water nuclear reactors include upper and lower tie plates supporting therebetween a matrix of upstanding fuel rods. These fuel rods are held in spaced apart relation and supported on a lower tie plate, and braced in a vertical upstanding relationship by an upper tie plate.
The tie plates have two functions. First, they support and/or brace the fuel rods. Second, they define intermediately of the support points for the fuel rods spatial intervals for the flow of coolant through the tie plates. In the case of the lower tie plate, spatial intervals are defined for permitting the inflow of liquid coolant. In the case of the upper tie plate, spatial intervals are defined for permitting the outflow of coolant in both the liquid and vapor form.
The fuel bundles are typically surrounded by a channel. This channel confines coolant flow between the tie plates. Thus the interior of the fuel bundle defines a steam generating path that is effectively isolated from the exterior of the fuel bundle.
When a nuclear reactor is operating, the upstanding fuel rods are within a thermal neutron flux. This flux is partially the function of the coolant-moderator (which is water) taking fast neutrons which are a product of the nuclear reaction and slowing these neutrons to thermal neutrons which continue the reaction. Thus, the coolant present within the fuel bundles contributes to the nuclear chain reaction producing energy within the nuclear reactor.
Finally, and for the purpose of combatting both rod bow and flow induced vibration, so-called fuel rod spacers are placed along the length of the fuel bundle. In present practice, six to seven evenly separated spacers define a matrix of separating material which individually surrounds each fuel rod and at the same time permits the upward substantially unobstructed flow of moderating coolant between the rods of the fuel bundle.